1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method of fabricating an LCD device, and more particularly, to an In-Plane Switching (IPS) mode LCD device and a method of fabricating an IPS mode LCD device.
2. Description of the Related Art
In general, an LCD device is formed by attaching an upper substrate and a lower substrate and injecting a liquid crystal into a space between the upper substrate and the lower substrate. In addition, a polarizer and a retardation film are attached on outer surfaces of the upper substrate and the lower substrate. Accordingly, by selectively arranging functional elements of the LCD device, an advancing direction or refractive index of light is changed, thereby forming an LCD device having high brightness and contrast characteristics.
A liquid crystal cell used in an LCD device may employ a twisted nematic (TN) operational mode. In the TN mode, light transmittance for displaying a gray scale is varied depending on a viewing angle, thereby limiting fabrication of large-sized LCD devices. To overcome the problem, an In-Plane-Switching (IPS) mode LCD device has been developed that uses a horizontal electric field, thereby improving viewing angle characteristics, such as a contrast, a gray inversion, and a color shift, as compared with the TN mode LCD device.
The IPS mode LCD device includes a pixel electrode and a common electrode formed on a common plane of a thin film transistor (TFT) array substrate, i.e., a lower substrate provided with a plurality of TFTs formed thereon. Accordingly, liquid crystal material is driven by the horizontal electric field formed by the coplanar pixel and common electrodes.
FIG. 1 is a plan view of an IPS mode LCD device according to the related art. In FIG. 1, an IPS mode LCD device includes a plurality of gate lines 111 and a plurality of data lines 113 crossing the gate lines 111 on a lower substrate. In addition, a TFT T is arranged at a cross point of the gate and data lines 111 and 113, wherein the TFT T includes a gate electrode 119, a source electrode 121, and a drain electrode 123.
A common electrode 115 and a pixel electrode 117 are disposed engaged with each other in a finger-type configuration on a unit pixel region defined by the gate and data lines 111 and 113. The finger-type common electrode 115 includes a first common electrode 115a having a plurality of vertical patterns and a second common electrode 115b that is a horizontal pattern for combining the plurality of vertical patterns as a single body. The finger-type common electrode 115 is spaced apart by a predetermined distance from the gate line 111. In addition, the finger-type pixel electrode 117 includes a first pixel electrode 117a having a plurality of vertical patterns and a second pixel electrode 117b that is a horizontal pattern for combining the plurality of vertical patterns as a single body. Both end branches of the first common electrode 115a formed within a unit pixel region overlap the data lines 113 on the data lines 113, thereby increasing an aperture ratio of the IPS mode LCD device.
FIGS. 2A to 2C are cross sectional views along I-I′ and II-II′ of FIG. 1 of a method of fabricating an IPS mode LCD device according to the related art. In FIGS. 2A to 2C, each pattern is formed by transferring a pattern of a mask on a substrate having a thin film formed thereon using a photolithographic process, which includes photoresist coating, align exposure, and developing.
In FIG. 2A, a conductive metal film is deposited onto a substrate 109, and then patterned to form a gate line 111 (in FIG. 1) and a gate electrode 119. Next, a gate insulating layer 118 is formed along an entire surface of the substrate 109 including the gate line 111 by depositing an inorganic insulator film, such as SiNx or SiO2, or an organic insulator film, such as acryl resin or benzocyclobutene (BCB).
In FIG. 2B, a pure amorphous silicon (a-Si) and an impurity-doped amorphous silicon (n+a-Si) are sequentially deposited onto the substrate 109 including the gate insulating layer 118, and then patterned to form an active layer 125 and an ohmic contact layer 127. Next, a conductive metal film is deposited onto the substrate 109 including the ohmic contact layer 127, and then patterned to form a data line 113, a source electrode 121, and a drain electrode 123. Then, a low dielectric material, such as BCB or an acryl resin, is deposited along an entire surface of the substrate 109 including the drain electrode 123, the source electrode 121, and the data line 113, and then patterned to form a drain contact hole 131 on the drain electrode 123.
In FIG. 2C, a transparent conductive metal film, such as indium-tin-oxide (ITO) or indium-zinc-oxide (IZO), is deposited and then patterned to form a finger-type common electrode 115 and a finger-type pixel electrode 117, which are spaced apart by a predetermined distance from each other and are engaged with each other. In FIG. 2C, vertical patterns 115a of the common electrode 115 and vertical patterns 117a of the pixel electrode 116 are shown.
However, since the substrate 109 is generally larger than a-size of the exposure mask used during the photolithographic process, the entire area of the substrate 109 is divided into a plurality of regions and is repeatedly exposed to light by a plurality of exposures by an exposure apparatus. Accordingly, misalignment between sequential exposures (i.e., stitch failure) occurs due to accuracy limits of the exposure apparatus, thereby deteriorating image quality of the IPS mode LCD device. Thus, the masks used to form the IPS mode LCD device become slightly distorted, thereby causing an overlay failure in which the gate electrode and the source/drain electrode are inaccurately overlapped on each of the pixel regions and deteriorating image quality of the IPS mode LCD device.
In addition, although the IPS mode LCD device shows improved viewing angle characteristics, such as a color shift, as compared with the TN mode LCD device, the IPS mode LCD device but does not maintain a uniform viewing angle with respect to all directions and cannot overcome the color shift problem.